Groundwood bleaching by hydrosulfite



FIPEEE]? Patented Sept. 28, 1948 i EFEREtlCE GROUNDWOOD BLEACHING BY f r HYDROSULFI'IE Howard J. Cronin, Glens Falls. N. Y., assignor to International Paper Company, New York, N. Y., a corporation of New York o No Drawing. Application December 1, 19-13 ,v I

I Serial No. 512,475 V 4 Claims. 1 It is an object of this invention to provide an improved bleaching process particularly applicable to the bleaching of ground wood pulp.

a It is a further object of this invention to provide an improved bleaching process involving the use of reducing or deoxidizing agents.

It is a further object of this invention to provide a bleaching process involving the use of a reducing or deoxidizing agent in which the bleaching effect will be substantially stable and not subject to retrogression or degradation with lapse of time. I I

For the purpose of this disclosure a reducing agent may be defined as any compound which will remove or tend to remove oxygen from chemical combination at temperatures below the boiling point of water and which will do so while in contact with oxygen compounds in an aqueous solution or suspension.

Heretofore it has been axiomatic in the art that bleaching accomplished by the use of reducing or deoxidizing agents is subject to the defect of being transitory, that is, the material to be bleached attains a considerable bleaching effect during contact with the active ingredient but'on subsequent exposure to air or even upon agitation or heating the material apparently reoxidizes, with a restoration of the original color andconsequent elimination of the bleaching effect. This inven-'- tion, while particularly applicable to the bleaching of ground wood pulp, is actually applicable to the bleaching of various materialsand, except as explicitly limited in the subdoined claims, the process is deemed to be applicable to any material, whether a papermaking pulpor a textilemaking fibre.

The term "emciency of the process as used herein relates to the cost of securing a given improvement of color and does not relate to the chemical efliciency of the reactions involved. A given increase in brightness is worth just so much money in saleability of the finished product. This sets a maximum limit upon the overall cost of producing the improvement. This overall cost is made up not only of the direct costof the bleaching agent or agents and the direct cost ofvlabor, but also of the depreciation of the equipment involved, the floor space and buildings occupied by such equipment, and the cost of pumping, and agitating the mixtures. conditions from the standpoint of cost efliciency may or may not have an relation to the optimum conditions for chemical efliciency.'

This disclosure deals primarily with bleaching cbtainedby the use of zinc or sodium hydrosul-.

i em-am: 2,; 1

Accordingly, the optimum phite, but it is to-be understood that-anyreducing agent compatible with the particular-material to be bleached may beemployed by the method herein set forth. This includes the hydrosulphites of various metals other than zinc or sodium, together with'various bisulphites and other reducing compounds. The selection of a particular 'reduc ing agent will be governed, first, by consideration of its compatibility-with the material to be bleached and, second, by considerationsj of cost, and suchselection is considered tube within the skill of the ordinary person skilled in the art. When certain materials are 'exposedrto the action of certain reducing compounds there may take place"side reactions which themselves will generate colored compounds detrimental to the desired bleaching efiect. The determination of whether such reactions willtake place as between :any particular material to be bleached and-any particular reducingagent is easilydeter'minable by routine tests. It isnot' necessary to determine the nature or even the existence vof such side reactions. It is only'necessary to determine whether an economic quantity of reducing agent will produce, first asuflicient improvement in brightness, and second asubstantially stable improvement, The essenceof this invention lies in intermixing the reducing agent and the material-to be bleached with a'minimum of agitation' and, immediately upon completion ofthe mixing step, maintaining the mixture in acondition of 'quiescence for a substantial period of time. Ithas been determined that if a sufficient periocLof quiescence follows the step of intermixingcarried out with minimum agitation, there' results; a bleaching efiect'which persists over considerable periods of time'with slight, if anydiminution. The periodduring which the mixture of stock and bleaching agent should remain quiescent fol lowing the mixlng'step is largely dependentupon the color improvement desired and this in" turn is, of course, dependent upon thev brightness of the stock before the bleaching step.; There will be a perceptible increase in brightness in as little as half an hour and the brightness will continue to increase measurably for as much astwentyfour hours. After five hours, however; the increase is reliativelysmall and quite slowly attained, so that as a commercial proposition it .will not usually be desirable'to provide a period "of quiescence longer than four to five hours. Ithasbeen determined that after four hours quiescence the mixture may be violently agitated without substantial decrease in brightness, althoughifyagitat/ion be resorted toaiter much less than four @EAQCH R0 hours there will be a diminution in brightness. Since in the handling of papermaking stock violent agitation is necessarily involved in supplying a the stock to the paper machine, diluting and running it over the wire, such steps should be deferred at leastethreeqhours fromqtheconclusion of the mixingstep, 1

In using zinc or sodium hydrosulphite it has been determined that the use of 1% of the hyr drosulphite by weight as against the dry weight of the pulp to'be bleached yields-substantiallyf better results than the use of but theuse' tional results over the use of 1%: Accordinglyg' the use of hydrosulphite in excess of 1% of dry tion, less agitation is required for sufficient mixing in the case of a liquid than in the case of a dry powder.

It is to be noted that in the use of this invention optimum conditions are encountered as dilu-tion increases. Thiszis counterr fiOzthB trend 'of ;devel01iment inall otheii;bleachingprocesses which under modern practice can be carried on at? greater concentrations.

weight of pulp is not recommended,..althoughsso far as the essence of this invention is concerned periodiofzquiesenceg Anrin'crea'seztin temperature frbmwofiffi to go rseems to1make littlxlifierence, but 'az'i increase to -1'00"=F; increases the? total Bimiprovement irrzbri'ghtness 'andzishortenscthe 'ti'met in -which:a;giveniimprovementzoccnrs;

"Tiiezconsistency of the stocmto besbleached is. also awfactor: Bleachingfitakes'zpl'acer-ineaiwater? suspehsion oi the: stocle'landfit'zhas z-zbeen four rd? thati dilution of thei suspensiod belbw 2' '(dry weig-tit of 'stock dividediby totalfiw'ei'ght oil 1 the suspension) deereeses the efflciency of -theiprocessg wliii -an increase- (515 consisteiricy beyond -5-% (similarly oalulated') equally= impairs" the effl ci'ency- M the process. Th'is' impairment of the effioiericy -oiithemrocess l-byzincreasing the' consistency beyond= 5% is belived tc belfdue -to-the fact that higher consistencies relquire ai greater amount 'o-f agitation for thorough -distribntion' of the' bleach-in'g agent; I "Theoretically; salmcst' no agitation'iwouldi be required iatifiiifiiiitefdilutibri and theretore the efil'ci'errcy of thei process would be greatest'at infl'r iite diluti'on'; T- h e praotical requirementsoi an op'enatin-g plant? howevemare responsiblefor settii'ig'z-"2% s assmi niniuml consistency? At sucltic'onsistency theslurry Wiltcbntaifi 39 ton's f water'fiofieach tori oi ifibreawlierm as zit- 1% consistency theresare'99 tons:fotcwater rom tonmt flbre,:.:andv t-lie1 probiemsiofistorage: midi-pumping make:such-extremei'diiutions un economical; -"It -has -i also :beenctdeterminedilthat me nest r'esixlts are f-secured -wheri 'the reduoin agentlin "dry powdredi-form isf zmixedi'lwith 'thestock; I? However; ai=io %.ssohition ofi-fthesblea'ch mg agent may:bezpresenvedrzionca lim-ited rtime (prefrably not morewthanotwoiortthreea hours) 1 undr' conditions of agitation and may -b'e used with "satisfactory' r'esuits; .c In practice} the convenience of lusi-ngzfa solution in' pl'atceoffhanoi -the liquid form oi bleeichiaigz-agentx In addi- Experiments have shown some improvement in OIbILfWhen-the pulp is adjusted to pH? prior to addition of the hydrosulphite. of 1 /2% contributes very little imthewayiofiaddr tocbe comniercia'llyfimportant to make such ad- However, such zadvantagefisivermslight and it is not considered mstmentllwcommercial o eration with ground pHo apprOXimately 5.5 the addition of the hydros'ulphite lowers the pH to a range of 4.8 to

1 Mimi-fits {6 -7321 501 It iias been found that entirely satisfactory results are secured within this range inasmuch asra pe rmanent' zincrease in.- brightness of at least 4 TIJOIIItS'LiSFDDUBI'iIIBCI' in;-a-:period,--of; three and-one-- half hourss. v V

From thestandpointioficostiefiiciency the optimumjoonditions underEwhich: this process: may be carried .iou-t are: ,5813; forth in the following;

example: V j

- (:1) Zincuhydrosulphite shouldbei-used in-an =(2)" Temperatureshould: be: maintained. at or.- slightly above F.

-(3Dri'I'i'ie:consistency ofithe suspension: should beenot less2than:2 /2"%:-nor more than'5%.

- 4)kflherhydrosulphite should be mixed withthee-suspensions With-ithe: least'possible; agitation. (5) ?Eollowi'ng-J the 1 mixing. step the mixture shouldwremain': quiescentforrnotiless than three- Grou'n wood-pulp Ewingan 1 initial brightnessi a: minimum ot agitatiomlzinc or sodium 'hyd'rosul phiteuii-anamountequal: to -1-%'"oi'the dry weight of th pulpa "Th'e 'pI-Iof the mixture dropped to 439. A temperature of-approximately 100 F. was maintained and the-mixture was allowed to remainouie'sc'ent-ifori three and "one-half hours. At the end of this period the brightness had risen to -63.7; showinga? gain of 4-.'l'- -points over the original' brightness; "ll ie bl'e'ached pulp was then formed mto 'paper by usual processes and the brightness of the paper thusformed was '64. Artera 'perie d of six 'weels during which samples oi tl'ie -paper- '--were stored in at desk drawer the Ir'1- a/nether -commercial' I operation a freshlygrotmd' spruee pulpmavi'ngan' initial brightness of- 66i6 "was 'treated substantially as set iorth in the receamg eXampIe and at' theend of three liours tlie brightness had risen'- 6.3 points to '7239. Attthe end ofi five hour's' the-brightness had' -risen the relatively-slight improvement 'in color which is attair'ied by 'prolongin'gthe period of quiescence substantially beyond three to three and'one-lialf hours-s is a-lsoi ill-llstrative "of theaprblicability mitt-he "present; proeess 'to -the bleaching' 'of-" a-- r'axampl is illustrativeof groundwood pulp of unusually high initial brightness.

The above detailed description and the specific examples herein set forth relate to preferred forms of the present invention. It is obvious that modifications and variations may be-resorted to by those skilled in the art within the scope of the following claims:

I claim:

1. A method of bleaching an aqueous suspension of groundwood pulp comprising: adding zinc hydrosulphite in an amount equal to 1% of the dry weight of pulp; mixing said zinc hydrosulphite with said suspension with a minimum of agitation and thereafter maintaining said mixture quiescent for not less than three hours.

2. A method of bleaching groundwood pulp comprising: making an aqueous suspension of pulp of consistency not less than 2 70 nor more than 5%; adding zinc hydrosulphite in an amount equal to not less than /2% nor more than 1 /2% of the dry weight of pulp; mixing said zinc hydrosulphite with said suspension with a minimum of agitation; and thereafter maintaining said mixture quiescent for not less than three nor more than five hours.

3. A method of bleaching an aqueous suspension of groundwood pulp comprising: adding zinc hydrosulphite in an amount equal to 1% of the dry weight of pulp; mixing said zinc hydrosulphite with said suspension with a minimum of agitation; thereafter maintaining said mixture quiescent for not less than three hours, and maintaining the temperature of said mixture at substantially 100 Fahrenheit throughout said period of quiescence.

4. A method of bleaching groundwood pulp comprising: making an aqueous suspension of pulp of consistency not less than 2 /2% nor more than 5%; adding zinc hydrosulphite in an amount equal to not less than /2% nor more than 1 5% of the dry weight of the pulp; mixing said zinc hydrosulphite with said suspension with a minimum of agitation; thereafter maintaining said mixture quiescent for not less than three nor more than five hours; and maintaining the temperature of said mixture at substantially 100 Fahrenheit throughout said period of quiescence.

HOWARD J. CRO-NIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 17,895 Roth July 28, 1857 1,027,953 Wickman May 28, 1912 1,645,061 Kress Oct. 11, 1927 1,662,951 Bragg Mar. 20, 1928 1,873,924 I-lirschkind Aug. 23, 1932 2,071,308 Hirschkind Feb. 16, 1937 2,199,376 Reichert Apr. 30, 1940 2,214,845 Wells Sept. 17, 1940 2,290,601 Reichert July 21, 1942 2,339,595 Williams Jan. 18, 1944 FOREIGN PATENTS Number Country Date 8,851 Great Britain A. D. 1904 442,336 Great Britain Feb. 6, 1936 CROSS REFERENCE SEARCH RQDM 

